As indicated in the parent application, the improved plasma gun disclosed therein finds application in a variety of environments for performing functions which either could not be performed previously, could not be performed well previously, or could only be performed without relatively large and expensive equipment. These functions include thrusters for satellite or other space station keeping and maneuvering applications, and the controlled generation of radiation at selected frequencies, generally within the extreme ultraviolet (EUV) band. The plasma guns disclosed for such applications were particularly advantageous in that they provided high reliability and pulse repetition frequency (PRF), and in particular a plasma gun having a PRF in excess of approximately 100 Hz and preferably a PRF in excess of 5,000 Hz for space applications and PRFs of at least 500 Hz and preferably 1,000 Hz for lithography or other applications requiring radiation generation.
In order to achieve these objectives, the plasma gun of the parent application had two general embodiments, one for space applications or other thruster applications, and a second embodiment radiation generator applications. In both cases, the plasma gun involved a center electrode and an outer electrode substantially coaxial with the center electrode, with a coaxial column being formed between the electrodes. A selected gas was introduced into the column through an inlet mechanism, and a plasma initiator was provided at the base end of the column. Finally, a solid state high repetition rate pulsed driver was provided which was operable on pulse initiation at the base of the column to deliver a high voltage pulse across the electrodes, the plasma expanding from the base end of the column and off the end thereof. For the thruster embodiment, the voltage of each of the pulses decreased over the duration of the pulse, and the pulse voltage and electrode length were selected such that the voltage across the electrodes reached a substantially zero value as the plasma exited the column. For this embodiment, the inlet mechanism preferably introduced the gas radially from the center electrode at the base end of the column, thereby enhancing plasma velocity uniformity across the column, plasma exiting the column for this embodiment at exhaust velocities which are currently in the range of approximately 10,000 to 100,000 meters per second, the exhaust velocity varying somewhat with application.
For the radiation source embodiment of the invention, the pulse voltage and electrode lengths are such that the current for each voltage pulse is at substantially its maximum as the plasma exits the column. The outer electrode for this embodiment of the invention is preferably the cathode electrode and may be solid or may be in the form of a plurality of substantially evenly spaced rods arranged in a circle. The inlet mechanism for this embodiment of the invention provides a substantially uniform gas fill in the column, resulting in the plasma being initially driven off the center electrode, the plasma being magnetically pinched as it exits the column, to produce a very high temperature at the end of the center electrode. A selected gas/element fed to the pinch as part of the ionized gas, through the center electrode or otherwise is ionized by the high temperature at the pinch to provide radiation at a desired wavelength. The wavelength is achieved by careful selection of various plasma gun parameters, including the selected gas/element fed to the pinch, current from the pulse driver, plasma temperature in the area of the pinch, and gas pressure in the column. The parent application for example indicates combinations of parameters for generating radiation at a wavelength of approximately 13 nm using for example lithium vapor as the gas fed to the pinch.
In order for the invention to function effectively in either of the above applications, it is critical that the preionization of the gas by the initiator provide an absolutely uniform preionization of the gas. For the parent application, this was achieved by forming holes evenly spaced around the column, with the gas either being introduced through the holes or directed at the holes. Electrodes were provided which were preferably mounted in the holes or otherwise at the base of the column, and preferably out of the column or closely adjacent thereto, which electrodes were fired to initiate plasma. The trigger electrodes were preferably evenly spaced around the base end of the column and were fired substantially simultaneously to provide uniform initiation of plasma at the base end, a DC signal being used to fire the electrodes. While this mechanism provides far more uniform plasma initiation than is possible with any prior arrangements, and is suitable for most applications, there are applications, particularly when the plasma gun is being used as a radiation source, where even more uniform plasma initiation is desirable. A need therefore exists for an improved plasma initiator providing such more uniform plasma initiation.
Further, while various gases/elements may be used with the plasma gun to provide a radiation source, each providing radiation at a number of different wavelengths, the energy/intensity at a given wavelength is roughly equal to 1/.lambda..sup.4 at high temperature. Thus, the lowest wavelength energy for a given element provides the highest energy output, and longer wavelengths have significantly lower energy, frequently many orders of magnitude lower energy. Therefore, to obtain maximum energy radiation from a given plasma gun, it is desirable to obtain most of the output at the lowest wavelength of the spectrum for the element ionized, which wavelength is obtained when the element/gas is in its single electron state. However, where a wavelength is desired which is not obtainable from an element in its single electron state, the number of wavelengths obtainable from element in this state being significantly restricted, particularly above 13.5 nm, then a technique must be provided for maximizing such wavelength in the output, and in particular to prevent such wavelength radiation from being overwhelmed by the far higher energy radiation, from the single electron state. Finally, it is desirable that radiation outputted from the plasma gun have as narrow an output angle as possible for certain applications.
A need therefore exists for an improved plasma gun and method for the use thereof which provides more uniform plasma initiation than is possible in prior art systems, including that of the parent application, which maximizes radiation at a desired wavelength for a wide range of radiations which may be obtained from the system and which permits narrowly focused beams to be obtained.